1. Field of the Invention
This invention relates to water-cooled nuclear reactors and more particularly to pressurized water reactors having direct vessel injection.
2. Description of Related Art
The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated from and in heat exchange relationship with a secondary side for the production of useful energy. The primary side comprises the reactor vessel enclosing a core comprised of a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer and pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump and a system of pipes which are connected to the vessel form a loop of the primary side. The primary side is also connected to auxiliary circuits, including a circuit for volumetric and chemical adjustment of the pressurized water. This auxiliary circuit, which is arranged branching on the primary circuit, makes it possible to maintain the quantity of water in the primary circuit by replenishing, when required, with measured quantities of water and to monitor the chemical properties of the coolant water, particularly its content of boric acid which is important to the operation of the reactor. During the periods when the chemical properties of the water are adjusted, it may be necessary to carry out tappings or injections into the primary circuit. Outside these periods of injections or tappings, the valves connecting auxiliary circuits, other than the circuit for volumetric or chemical control to the primary side, are closed. The primary side is then theoretically isolated and completely sealed, with the result that the quantity of water in the primary side is theoretically constant.
In practice, however, it is observed that this quantity of cooling water diminishes during the operation of the reactor, as a consequence of unavoidable leaks. It is important to maintain the level of coolant within the core, and when makeup water is required, in some nuclear reactor system designs it is directly injected through direct vessel injection nozzles into the reactor vessel in the downcomer between the reactor vessel and the core barrel. A deflector attached to the core barrel in line with the direct vessel injection nozzle directs the incoming water down the downcomer to the bottom of the vessel where it changes direction and is directed up through the lower core support plate to the core. The incoming water entering through the direct vessel injection nozzle is at approximately 50° F. (10° C.). Prior to a direct vessel injection transient, the direct vessel injection flow deflector has a uniform temperature consistent with the reactor cooling system cold leg temperature, which is approximately 535° F. (279° C.). At the onset of the direct vessel injection transient, the direct vessel injection flow deflector interior surfaces cool almost instantaneously to 50° F. (10° C.). As a result of the direct vessel injection transients, the flow deflector experiences a significant amount of contraction because of rapid cooling. For existing plants having direct vessel injection, the flow deflector can best be described as a (solid) rectangular plate or block with a machined groove extending just below a top lip (74) to the bottom of the plate. The machined groove turns the direct vessel injection flow down into the reactor pressure vessel core barrel annulus between the pressure vessel and the core barrel.
Since the flow deflector can be characterized as a solid block, one side of the weld (that is, the “heel” of the fillet weld) is constrained to follow the contraction of the block surface of the deflector. The “toe” of the weld remains in contact with the outside diameter of the core barrel. The average temperature of the core barrel, approximately 580° F. (304° C.), is significantly higher than the direct vessel injection flow deflector during the direct vessel injection transient. Therefore, during the direct vessel injection transient, the “throat” of the weld experiences a significant amount of “shear action.” This “shear action” is necessary to accommodate the differential strain between the direct vessel injection flow deflector and the core barrel. As a result, high stresses are anticipated for the weld.
New reactor designs, such as the AP1000 reactor design offered by Westinghouse Electric Company LLC, anticipate an increased number of occurrences of direct vessel injection system transients. The direct vessel injection connections are used to reduce the side effects of accidents caused by reactor coolant system pipe breaks. In plants that do not use direct vessel injection connections, core make-up water is introduced through the cold leg piping. A break of a main coolant loop pipe will cause spillage of safety injection flow. For the AP1000 with direct vessel injection, breaks of the main coolant loop piping will not cause any safety injection spillage.
Previous plants that used direct injection nozzles were two loop plants that used them only for safety injection. The direct vessel injection nozzles on the AP1000 are connected to the core makeup tank for safety injection and to the in-containment refueling water storage tank drain lines, accumulators, and shut down cooling pumps. These additional connections add significant transients to the direct vessel injection nozzle and deflector. These transients are anticipated to set up high stresses that can result in an unacceptable fatigue life of the fillet weld attaching the flow deflector to the core barrel. The current weld design attaching the flow deflector to the core barrel will not likely accommodate the relative differential expansion of the flow deflector and core barrel over an increased number of injection transients, because of the inherently “stiff” characteristics of the mating surfaces.
Accordingly, a new flow deflector to core barrel interface is desired that can better accommodate the stresses set up by the rapid cooling of the flow deflector during a direct vessel injection transient.